1. Technical Field
The present invention relates generally to orthogonal frequency division multiplexing (OFDM) demodulation technology and, more particularly, to technology for estimating an OFDM integer frequency offset.
2. Description of the Related Art
OFDM signals have been widely used as physical layer standard technology in high-speed wireless communication standards, such as WiFi, Long Term Evolution (LTE), IEEE 802.16, etc. due to their advantages of being robust to multipath fading and achieving high frequency efficiency.
OFDM signals perform modulation and demodulation in the frequency domain, and particularly transmit information on their sub-carriers.
Due to the instability of an oscillator on a reception side or the Doppler effect, a frequency offset occurring in a carrier is indispensible. This frequency offset becomes an absolute cause of error during the demodulation of an OFDM signal because the OFDM signal encodes information in the frequency. Unless a frequency offset is accurately estimated and compensated for, an OFDM symbol is shifted by the frequency offset in the frequency domain, so that the orthogonality of a carrier is destructed, thereby making demodulation impossible or resulting in demodulation into a symbol having completely different information.
A frequency offset is classified as an integer frequency offset that is represented by an integer multiple of a sub-carrier spacing, or a fractional frequency offset that is represented by a value within a sub-carrier spacing. An integer frequency offset and a fractional frequency offset are independently estimated because they have different influences on an OFDM signal and are estimated using different estimation methods. Generally, a fractional frequency offset is estimated first, and then an integer frequency offset is estimated by compensating for the fractional frequency offset.
In general, the estimation of and compensation for an OFDM frequency offset are performed using training symbols that have been made known to both a transmission side and a reception side in advance.
A transmission side generates an OFDM training symbol via an inverse fast Fourier transform (IFFY) and transmits it. If it is assumed that a fractional frequency offset has been completely estimated and compensated for using a well-known method, a reception side estimates an integer frequency offset based on a frequency offset candidate value related to the acquisition of the largest correction result by performing a correlation operation on previously known local training symbols and local symbols shifted using a plurality of frequency offset candidate values, with respect to a received OFDM symbol.
In this case, the conventional technology of the following non-patent document [1] is problematic in that it cannot correctly estimate an integer frequency offset when there is a time offset. Although the conventional technology of non-patent document [2] can overcome the influence of a time offset, it is problematic in that it requires highly complex operations.
Accordingly, there is a need for a new method of estimating an integer frequency offset, which is capable of eliminating high complexity, i.e., the problem of conventional frequency offset estimation schemes, while overcoming the influence of a time offset.